C4 photosynthesis—evolution or design?

Life depends on photosynthesis, where plants take carbon dioxide from the atmosphere
and ‘fix’ it into high-energy sugars using light as the energy source.
Two basic forms of photosynthesis have been discovered. In one, the first compound
made from CO2 is a three-carbon compound, so this is called C3
photosynthesis. In the other, the first compound is a four-carbon compound, so it
is called C4 photosynthesis.1
Most plants are C3; about 15% of species have the C4 system.
Examples of C3 plants include wheat, rice, potatoes and cabbage. C4
plants include maize, sugar cane, sorghum and succulents—mainly tropical/arid
environment species.

C4 and C3 plants differ in their leaf anatomy and where photosynthesis
occurs. C3 plants have chloroplasts throughout the internal (‘mesophyll’)
leaf cells, and there are air spaces around the cells to allow ready diffusion of
CO2 into them. In C4 plants, the photosynthetic cells cluster
around the vascular bundles (leaf veins) and there are no air spaces around the
photosynthetic cells. The photosynthetic cells are called bundle sheath cells
because they form a tight sheath around the vascular bundles.

The Calvin-Benson Cycle of photosynthesis. Each turn of the cycle produces a molecule
of phosphoglyceraldehyde ‘PGAL’, (containing 3 carbon atoms). This is
transported from the chloroplast to make glucose and fructose, which in turn condense
to form sucrose.

C3 and C4 plants share the same light-harvesting systems,
as well as the same enzyme cycle for incorporating the carbon into sugars—the
Calvin-Benson cycle. The first enzyme in this cycle, nicknamed ‘Rubisco’,
makes up 25% of the protein in leaves, which makes it the most abundant protein
on Earth. Rubisco takes CO2 and adds it to a 5-carbon sugar, making two
3-carbon
sugar molecules.

C4 plants have extra enzymes operating in the leaf. These incorporate
the CO2 (actually bicarbonate, HCO3–) into
a
4-carbon
compound (usually malate), which the mesophyll cells transport into the bundle sheath
cells via many tiny tubes called plasmodesmata. Here another enzyme releases the
CO2 for Rubisco to fix into sugars in the same manner as in C3
plants. The bundle sheath cells have specialized thickened cell walls and they have
no air spaces around them, so the CO2 cannot escape and it becomes concentrated
to at least 10 times that of normal outside air. This accounts for one of the major
differences between C3 and C4 plants: in the short term, C3
plants increase their rate of photosynthesis in response to increased atmospheric
levels of CO2, but C4 plants don’t.

C3 and C4 plants also differ in that C3 plants
exhibit ‘photorespiration’, where they lose some of the CO2
fixed into
3-carbon
sugar, whereas C4 plants don’t. This happens because O2
competes for the active site on Rubisco to which CO2 binds. While Rubisco
has a much greater affinity for CO2, the partial pressure of O2
in air is 700 times greater than that of CO2. Oxygen drives the release
of CO2 with the production of the energy-depleted forms of energy-carrier
molecules (ADP and NADP).

This seems to be a safety mechanism to avoid damage to the photosynthesis system
at low CO2 levels. If there is inadequate CO2 to fix the energy
harvested by the chlorophyll system, then oxygen radicals form and these damage
the light harvesting system. Photorespiration maintains a supply of ADP and NADP
to accept the energy generated by the light-harvesting system.

C4 plants concentrate their CO2, thus suppressing photorespiration.
Also, since the supply of CO2 is maintained, even at low concentrations,
there is always a sink for the energy from the light harvesting, so damage to the
photosystems is avoided. So there is no need for photorespiration.

Why two methods of fixing CO2?

Why do C3 plants tend to be temperate in their adaptation and C4
plants tropical/arid? The rate of photorespiration rises rapidly with temperature,
so it becomes a much more serious problem, in terms of its ‘inefficiency’
(loss of fixed carbon), in the tropics. On the other hand, the C4 system
has energy costs: each CO2 fixed into malate needs one NADPH and one
ATP for the complete cycle. So the relative advantages seem to be due to the trade-off
between photorespiration in C3 plants and the extra costs of carbon fixation
in C4 plants. With increasing temperatures, the cost of photorespiration
becomes greater than the extra cost of the C4 system, which is met by
the increased sunlight energy anyway, so the latter prevails.

C4 plants also do well in arid environments. In this situation the plant
closes its stomata (leaf pores) to conserve water. This also reduces the amount
of CO2 entering the leaf and raises the leaf temperature. The enzyme
that fixes CO2 in C4 plants has a much greater affinity for
CO2 than Rubisco, which does the job alone in C3 plants. So
C4 plants are still able to supply plenty of CO2 to the Rubisco
in the photosynthetic cells, whereas a C3 plant would have trouble.2

The origins of the C4 system

Some 8,000 to 10,000 species of plants in 18 families, including both monocots (which
includes grasses) and dicots (roughly, ‘broad-leaved’ plants), have
the C4 system. C4 metabolism has even been found in a single-celled
marine diatom.3

Many flowering plant families have both C3 and C4 species.
Some species are intermediate, showing both C3 and C4 characteristics.
In the Atriplex genus, some species are C3, while others are
C4, and C3 and C4 species have been hybridized.4 Wood and Cavanaugh have reviewed
the genus Flaveria, which has species of C3, C4 and
intermediate type, many of which hybridize.5

The distribution of C4 species does not form any pattern that could relate
to any reasonable evolutionary phylogeny.

The distribution of C4 species does not form any pattern that could relate
to any reasonable evolutionary phylogeny. Consequently, evolutionists have proposed
that C4 photosynthesis has arisen independently at least 30 times—a
classic case of ‘polyphyletic evolution’.

However, C4 chemistry involves several complex enzyme systems and the
chemistry is remarkably consistent across the spectrum (there are three types of
enzyme used to release the CO2 from the organic acid that transports
the CO2, otherwise the chemistry is similar).

To believe that C4 chemistry arose once by natural processes would require
super ‘faith’ for the evolutionist. But to propose that such a system
with its new complex coded genetic information arose separately some 30 times by
mutations and natural selection, and that these processes arrived at essentially
the same solution, stretches credulity to breaking point. This would be an extreme
example of ‘convergent evolution’—even more than the supposed
polyphyletic origin of the eye in general and the compound eye in particular.6

Some species that exhibit both C3 and C4 forms are even able
to switch from one to the other during development. This suggests that maybe the
C4 chemistry is latent in C3 plants, or is suppressed by some
means. In the marine diatom mentioned above, C4 metabolism seems to be
facultative.3

Wood and Cavanaugh5 concluded from their baraminological study of Flaveria
that the C4 photosynthetic pathway arose from plants that were originally
C3, and this probably happened post-Flood. These authors propose that
the genetic information for C4-mode photosynthesis was present in the
original created kinds, but latent and has become activated since.

Surprise: C3 plants have the C4 system!

Diagrammatic representation of the Hatch-Slack system of CO2 capture
and fixation that operates in the roots and stems of C3 plants, which
were thought to lack this capacity entirely. (Xylem and phloem are actually in vascular
bundles together; not separated as in the diagram.)

Now Hibberd and Quick have shown that tobacco and celery, two classical C3
plants, contain virtually all the C4 characteristics, not in their leaves,
but in their roots, stems and petioles.7
They showed that CO2 respired in the roots is fixed into malate by the
same enzyme that fixes CO2 in the leaves of C4 plants. The
malate moves in the xylem stream up the plant where it transfers into bundle sheath
cells surrounding the vascular bundles in the stems and petioles. Here all three
decarboxylation (CO2-releasing) enzymes identified in the three sub-types
of the C4 system are present in elevated levels. They release the CO2
so that Rubisco can use it in the Calvin cycle. The chemistry is apparently identical
to the C4 system. These plants differ from C4 plants only
in the site of synthesis of the malate (roots in C3 plants versus leaf
mesophyll cells in C4 plants) and its transfer to the bundle sheath cells.
Even the anatomy of the bundle sheath cells in the stems and petioles is similar.

This makes for a very efficient system for retrieving respired carbon from the roots.
Indeed, CO2 may even enter the roots from the soil, where the level of
CO2 is usually quite high due to the activity of heterotrophic micro-organisms.
This would reduce the CO2 concentration in the soil, which would be beneficial
to the aerobic organisms living there. What wonderful design for an efficient ecology!

Hibberd and Quick point out that since so much of the C4 system is already
present in the C3 plants, ‘fewer modifications are needed for C4
photosynthesis to evolve’. Indeed, are we talking about the origin of new
complex, coded genetic information at all, or are we looking at adaptation based
on existing genetic information—as proposed by the creationists Wood
and Cavanaugh?

It now seems that the genes for C4 enzymes and anatomy are selectively
expressed in the roots, stems and petioles of C3 plants, but are suppressed
in the leaves. C4 plants differ in having these genes expressed in the
leaves as well. If the suppression in the leaves of C3 plants were due
to the synthesis of proteins that interact with promoter sequences, for example,
it may even be possible to see mutations in the genes for these proteins that result
in the expression of C3–C4 or C4 photosynthesis.
Or there might be some designed means of switching on this adaptation genetically
so that it is inherited once switched on—something like Wood’s Altruistic
Genetic Elements (AGEs)?5

These developments underline just how cleverly the original plants were created—with
built-in latent capacity for adaptation to a wide range of environments. It will
be interesting to see the details fleshed out.

References

The basic details of C4 photosynthesis were elucidated
by Australian scientists in the 1960s. See Hatch, M.D. and Slack, C.R., Photosynthesis
by sugarcane leaves, Biochem. J.101:103–111, 1966.
Return to Text.

A variation on the C4 theme is seen in CAM (crassulacean
acid metabolism) plants. Typically succulent desert plants, they open their stomata
at night to fix CO2, storing the fixed form in vacuoles (reservoirs within
cells), then release the CO2 for photosynthesis during the day
when the stomata shut. In this manner they conserve water very efficiently.
Return to Text.

Oakley, T.H. and Cunningham, C.W., Molecular phylogenetic
evidence for the independent evolutionary origin of an arthropod compound eye, Proc.
Nat. Acad. Sci. USA99(3):1426–1430, 2002. Their
abstract says, ‘These results illustrate exactly why arthropod compound eye
evolution has remained controversial, because one of two seemingly very unlikely
evolutionary histories must be true. Either compound eyes with detailed similarities
evolved multiple times in different arthropod groups or compound eyes have been
lost in a seemingly inordinate number of arthropod lineages.’
Return to Text.

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